1. Field of the Invention
The present invention relates to a method for improving the imaging performance in a photolithographic system having a pupil plane and using a phase shift mask.
2. Related Art
The ability to fabricate integrated circuit (IC) chips having devices with progressively smaller feature sizes so as to support increasingly larger densities depends upon continual evolution of photolithographic methods. On an IC chip, devices and their connections are typically fabricated in phases. Several phases include processes that modify portions of a semiconductor substrate (i.e., a wafer). For each of these phases, the portions to be processed must be isolated from the remaining portions of the wafer. Often this is accomplished by applying a layer of film (i.e., photoresist) on a surface of the wafer and exposing the photoresist to a pattern of light. The pattern distinguishes the portions of the wafer to be processed from the remaining portions. The pattern of light typically is produced by causing light to pass through a mask (i.e. a reticle) upon which the pattern is formed as opaque and transparent portions. Where light passes through the transparent portions of the reticle, corresponding portions of the photoresist are exposed. Either the exposed or unexposed (but not both) portions of the photoresist are removed to reveal the underlying portions of the wafer to be processed. The remaining portions of the wafer are protected from the process by the remaining photoresist.
Machines that cause light to pass through a reticle to expose photoresist on a wafer are referred to as wafer steppers or wafer scanners. In order to achieve an accurate representation of the reticle pattern at submicron dimensions on the photoresist, it is necessary to use a light source that can support both a high degree of resolution and depth of focus. This requirement has led to the use of lasers as light sources for photolithographic applications.
Ironically, the challenge to increase the density of devices fabricated on an IC chip is frustrated by the same smaller feature sizes upon which a greater density depends. Smaller pattern dimensions on the reticle, particularly for linewidths, cause greater diffraction of light passing through the pattern. At the wafer, this greater diffraction of light can manifest itself as “spillover”, whereby the distribution of electromagnetic energies from two adjacent features merge together so that it is difficult to distinguish one feature from the other.
However, by using a phase shift mask, the distribution of electromagnetic energies from two adjacent features are out of phase with each other. Because the intensity of the light is proportional to the square of the vector sum of the amplitudes of electromagnetic energies, the use of a phase shift mask increases the likelihood that there will be a point of minimum intensity between the two adjacent features so that the can be distinguish one from the other.
Furthermore, by using a phase shift mask, the half order lights are the directions of constructive interference rather than the zero order and first order lights as are used in traditional photolithographic systems. Using the half order lights allows the spacing between features on the reticle to be reduced. Reducing the spacing between features on the reticle increases the angle of diffraction of the half order lights. The angle of diffraction can be increased so long as the half order lights are captured by the conditioning lenses of the photolithographic system.
Unfortunately, realizing a viable phase shift mask depends upon an ability to precisely fabricate recesses (or rises) in (on) the reticle having a depth (height) of an odd multiple of one half of the wavelength of the light, a specific width, and an accurate spacing from adjacent features on the reticle. Deviations from these criteria can give cause the half order lights not to be completely out of phase with each other so that the zero order light is not completely canceled by destructive interference. This phenomenon is referred to as “zero order leakage”.
Zero order leakage can cause variations in the intensity of the light that exposes photoresist that is above or below the nominal focal plane. These variations in intensity, in turn, can cause variations in the linewidths formed on the wafer. Such variations in the linewidths formed on the wafer can have a detrimental effect on the electrical or electronic characteristics of the device being fabricated.
What is needed is a method of preventing zero order leakage from causing variations in the intensity of the light that exposes photoresist. Preferably, such a method should be easily implemented and inexpensive.